Field of the Invention
The present invention relates to a process for preparing esters of acrylic or methacrylic acid [(meth)acrylic acid]. (Meth)acrylic esters are generally produced in industry by esterification of (meth)acrylic acid with alkanols in the presence of strong acids as esterification catalysts (e.g. sulfuric acid, phosphoric acid, alkanesulfonic acids, arylsulfonic acids or cation exchangers having sulfonic acid groups). Such processes are known for example from Kirk-Othmer, Encyclopedia of Chemical Technology, Vol. 1, pages 347-348.
As well as the desired ester and optionally an entrainer, the esterification mixture also includes unconverted starting materials (alkanol, (meth)acrylic acid) and the catalyst and also the corresponding alkyl acetate and dialkyl ether as low boiling by-products. The alkyl acetate is formed from the acetic acid which is always formed as a by-product in the synthesis of acrylic acid by propylene oxidation or from acetylene by the Reppe process, and is very difficult to separate from the acrylic acid, as known for example from EP-A 551 111 and GB-B-1 120 284. It is well known that ethers are formed from alkanols in the presence of strong acids.
The esterification mixture is generally worked up in multiple stages:
1. Extraction of the catalyst with water (known from EP-A-618 187) and/or neutralization with an aqueous base (known from EP-A-566 047 and EP-A-609 127). PA0 2. Wash with water and alkanol removal (known from EP-A-566 047, col. 1, 1. 19-21). PA0 3. Distillative removal of low boilers (e.g. acetate, ether, unconverted alkanol) and any entrainer used (known from U.S. Pat. No. 2 917 538, col. 1, 1. 61-68). PA0 4. Distillative removal of (meth)acrylate from high boilers (e.g. oligomers, polymers, Michael addition products, polymerization inhibitors).
However, the processes recited under 1 and 2 in particular have the following disadvantages:
The acids used as catalysts, and any of their esters which are formed, have to be removed from the reaction mixture before further processing. Generally, this is accomplished by washing and neutralizing the reaction mixture with alkali or alkaline earth metal hydroxide or carbonate solutions. The resulting wastewaters are difficult to dispose of benignly. If sulfuric acid is used as catalyst, it will react with the alkanol in question to form chiefly its monoester, as mentioned. The salts of sulfuric monoesters, especially the salts of the esters with higher alkanols, are surface-active and would, on disposal, appreciably impair the quality of t he wastewaters from the process and cause a considerable loss of product of value.
Furthermore, direct neutralization gives rise to a layer of sludge, which greatly impairs the separation between ester phase and water phase (EP-A-566 047, col. 1, 1. 49ff). A very complicated neutralization with bases of different strengths is proposed to solve this problem (ibidem).
Recovery of the catalyst is thus desirable for ecological and economic reasons. Appropriate processes are known. For instance, CZ-B-179 808 discloses a process for recovering mineral acids from esterification mixtures by extracting the esterification mixture with water, concentrating the aqueous phase by distillation and recycling the resulting concentrated aqueous catalyst solution into the esterification reaction. However, this process is energy-intensive.
EP-A-0 618 187 (= U.S. Pat. No. 5,386,052) describes a process for preparing (meth)acrylic esters by extracting the catalyst with water and recycling the extract into the esterification reaction, optionally after distillative concentration. However, it is emphasized that sulfuric acid is not suitable for use as catalyst by reason of the poor extractability of the monoalkyl sulfate, since the large amount of water which would be necessary to ensure adequate extraction of the monoalkyl sulfate would affect the esterification reaction adversely. The catalyst used is therefore selected from alkyl- or aryl-sulfonic acids (column 2, line 55ff), which, however, are significantly more expensive than sulfuric acid.
Following catalyst removal or neutralization of the acids, the esterification mixture is saturated with water (1 to 2% by weight, based on the esterification mixture). Before extraction or neutralization, the esterification mixture contains only about 0.1% by weight of water, based on the reaction mixture. However, the economically desirable isolation of the alkanol for recycling into the esterification process cannot take the form of a simple distillative removal of the alkanol from this water-saturated mixture owing to the formation of binary and ternary azeotropes. Such azeotropes are illustrated below in Table 1 for butanol.
TABLE 1 % by weight, based on bp. at Azeotrope azeotropic mixture 1 bar water/butanol/butyl acetate 27.7%/10.7%/61.6% 89.9.degree. C. water/butyl acetate 27.3%/72.7%/ 90.3.degree. C. water/butanol/butyl ether 29.9%/27.8%/42.3% 90.4.degree. C. water/butanol/butyl acrylate 39.5%/28.9%/31.6% 92.9.degree. C. water/butyl ether 33.9%/66.1% 93.1.degree. C. water/butanol 48.0%/52.0% 93.5.degree. C. water/butyl acrylate 39.7%/60.3% 94.4.degree. C.
A direct recycle of the butanol fraction would thus always also mean having to recycle water, which would have an unfavorable effect on the esterification equilibrium. In addition, the alkyl acetate or dialkyl ether formed would also be recycled, which would lead to an undesirable continuous enrichment of this ester or ether in the esterification mixture.
An elaborate distillative separation of water, alkanol, acetic ester, ether and target ester is therefore required to prevent losses of products of value.
In addition, owing to the relatively good water solubility of some of the alkanols (e.g. butanol, about 7% at 20.degree. C.), considerable amounts of unconverted alkanols will also always be extracted with water in the course of the neutralization of the remaining acids and/or the subsequent wash. This makes it necessary to provide an additional workup for the aqueous phase (by stripping, for example) to recover the alkanols.